1. Field of the Invention
The present invention relates to double-clad optical fibers and devices utilizing double-clad optical fibers and, more specifically, to double-clad optical fibers having an inner cladding with a high numerical aperture and endoscopes utilizing such double-clad optical fibers.
2. Technical Background
Endoscopes, specifically non-linear optical endoscopes utilizing two-photon fluorescence processes have emerged as one of the best, non-invasive means of performing fluorescence microscopy on a sample and/or a subject. The non-linear optical endoscopes utilize a source or excitation laser beam directed onto a target with an optical fiber. The interaction of the excitation laser beam with the target causes the target material to fluoresce or emit visible light which is collected back into the endoscope with the optical fiber for further analysis, imaging, spectroscopy and the like. Compared with single-photon microscopy, two-photon fluorescence microscopy offers inherent optical sectioning properties, greater penetration depths and flexible optical spectra accessibility.
One of the key components of non-linear optical endoscopy utilizing the two-photon fluorescence process is the optical fiber employed in the endoscope. While standard single mode optical fiber may be utilized in the endoscope to deliver the excitation laser beam to the target of interest, these optical fibers are not suitable for collecting the optical signals emitted by the target back into endoscope for further analysis, image formation, spectroscopy and the like. This is primarily due to the low numerical aperture and small core size of standard single mode fibers which limits the collection efficiency of the optical fibers.
To improve the collection efficiency of the non-linear optical endoscope, double-clad optical fibers may be used. Such optical fibers generally include a core, an inner cladding, and an outer cladding. The excitation beam of the endoscope is delivered to the target via the core of the optical fiber and the light emitted from the target is collect back into the endoscope via the inner cladding of the optical fiber. However, the collection efficiency of most standard double-clad optical fibers is still low due to the low numeric aperture of the inner cladding. Development of double-clad optical fibers having inner claddings with greater numerical apertures, such as photonic crystal double-clad optical fibers has been developed. However, the photonic crystal double-clad optical fibers are generally more difficult to manufacture than standard optical fibers and, as a result, expensive.
Accordingly a need exists for alternative double-clad optical fibers and non-linear optical endoscopes employing the same
In one embodiment, a double-clad optical fiber includes a core, an inner cladding and an outer cladding made of silica-based glass. The core has a first index of refraction n1 and does not contain any active rare-earth dopants. The inner cladding may surround the core and includes a radial thickness of at least about 25 μm, a numerical aperture that is greater than the numerical aperture of the core, and a second index of refraction n2 such that n2<n1. The relative refractive index percent (Δ%) of the core relative to the inner cladding may be greater than about 0.1%. The outer cladding may surround the inner cladding and include a radial thickness greater than about 10 μm and a third index of refraction n3 such that n3<n2. The relative refractive index percent (Δ%) of the inner cladding relative to the outer cladding in some embodiments may be greater than about 1.5%.